The invention relates to a valve timing control device as a hydraulic actuator mounted on an end of a camshaft, which modifies timing for the opening and closing of both or one of intake and exhaust valves depending on conditions when an engine is operated.
A vane-equipped or helical piston-equipped valve timing control device is known as a conventional hydraulic valve timing control device. The device is arranged between a timing chain or chain sprocket and a camshaft, the timing chain or chain sprocket defined as a valve-driving system rotating in synchronization with a crankshaft of an engine to drive the camshaft. Oil derived from an oil pump is controllably supplied to the valve timing control device and discharged to outside, by way of an oil control valve (hereafter, referred as an OCV). In this way, it is possible to modify relatively angular displacements of the camshaft with respect to those of the crankshaft. When the angular displacement of the camshaft is variably controlled in advanced or retarded direction, it is possible to optimize timing for the opening and closing of an intake or exhaust valve depending on the number of revolutions and loads of the engine. As a result, it is possible to reduce exhaust gas, to improve power and to increase gas mileage.
An actuator known as the hydraulic valve timing control device includes the vane-equipped and the helical piston-equipped devices. In the vane-equipped device, a plurality of hydraulic chambers is comprised of a vane-equipped rotor and a housing element accommodating the rotor and allowing rotation in a required range. Oil derived from the oil pump is controllably supplied to the hydraulic chambers and discharged to the outside, by way of the OCV. In this way, the hydraulic pressure is changed to shift angular displacement of the camshaft with respect to the crankshaft to advanced or retarded position. On the other hand, the helical piston-equipped device includes a first helical gear formed at a hydraulic piston moved reciprocally in an axial direction due to a hydraulic pressure derived from the OCV and a second helical gear engaged with the first helical gear. These gears are rotated in a required range on the basis of twisting of a helical spline in a housing element. In this way, it is possible to shift angular displacement of the camshaft with respect to the crankshaft to advanced or retarded position. In either case, timing for the opening and closing of an intake or exhaust valve is controlled due to the hydraulic pressure. For example, JP-A-92504/1989, JP-A-121122/1996, JP-A-60507/1997 and JP-A-280018/1997 are known as the former vane-equipped valve timing control device.
Especially, with an exhaust valve timing control device, a driving force derived from a crankshaft of the engine however exerts a force in lo a retarded direction on a camshaft. Moreover, at a time when the engine is started and so on, a pump of the engine is not yet actuated, and the hydraulic pressure is not functioned. Under the conditions, with the conventional device, the camshaft is rotated in the retarded direction when normal advance control cannot be performed due to the force in the same direction. As a result, timing for the opening of the exhaust valve is delayed to lead instability of idling such as a deterioration of starting characteristics of the engine. To solve the problems, a biasing means is arranged in the valve timing control device. Under the conditions of the engine that the hydraulic pressure is not functioned at a time when the engine is started, the biasing means biases the camshaft in the advanced direction against the force in the retarded direction exerted on the camshaft by the driving force derived from the crankshaft. In this way, the engine is started with stability. For example, JP-A-68306/1998 and JP-A-264110/1997 are concerned with the conventional device above.
The former gazette JP-A-68306/1998 discloses a device including a rotor rotatable in synchronization with a camshaft, a biasing means biasing the rotor to rotate a camshaft in an advanced direction with respect to a crankshaft, and a lock mechanism which allows to lock the rotor. With the device, the biasing force of the biasing means is set to be larger than the maximum torque on starting the engine and be larger than an average torque.
The latter gazette JP-A-264110/1997 discloses a device including a vane constituting a plurality of hydraulic chambers formed at inner peripheral sections of the device, and a biasing means biasing a camshaft so as to avoid opening both intake and exhaust valves at the same time. With the device, the biasing force of the biasing means is set to be smaller than a hydraulic pressure supplied to and discharged from the hydraulic chambers. When the hydraulic pressure is reduced, the biasing means also biases the camshaft in advanced direction.
FIG. 1 is a radial or lateral cross sectional view of an internal structure o f a vane-equipped device disclosed in the gazette JP-A-68306/1998. In the drawing, a reference numeral 100 denotes a shoe-equipped housing defined as a driving force transferring member and 101 denotes a vane-equipped rotor defined as the driving force transferring member rotatably arranged in a required range of the shoe-equipped housing 100. Shoes 100a, 100b and 100c projected inwardly in a radial direction are arranged at an inner peripheral section of the shoe-equipped housing 100. Vanes 101a, 101b and 101c projected outwardly in the radial direction are arranged at an outer peripheral section of the vane-equipped rotor 101. The shoes 100a, 100b and 100c and the vanes 101a, 101b and 101c partition a space between the shoe-equipped housing 100 and the vane-equipped rotor 101 into a plurality of rotor-retarding side hydraulic chambers 102, 103 and 104 and rotor-advance side hydraulic chambers 105, 106 and 107. Recesses 108 are formed at the shoes 100a, 100b and 100c facing the rotor-advance side hydraulic chambers 105, 106 and 107, respectively. Recesses 109 are formed at the vanes 101a, 101b and 101c facing the rotor-retarding side hydraulic chambers 102, 103 and 104, respectively. In each rotor-advance side hydraulic chamber 105, 106 or 107, spring members 110 defined as a biasing means are arranged between both recesses 108 and 109. The shoe-equipped housing 100 is mounted rotatably on an exhaust camshaft corresponding to the exhaust valve and the vane-equipped rotor 101 is fixedly joined at an end of the exhaust camshaft with bolts so as to be rotated in synchronization with the exhaust camshaft.
Next, an operation will be explained.
First, a rotational driving force derived from a crankshaft (not shown) of the engine is transferred to the exhaust camshaft (not shown) by way of a timing chain or timing belt (hereafter, a driving force transferring means, not shown in either of the cases), the shoe-equipped housing 100 and the vane-equipped rotor 101 having a chain sprocket (not shown) or a timing chain (not shown) and defined as a driving force transferring member.
When the valve timing control device is actuated, the vane-equipped rotor 101 is rotated relative to the crankshaft 1 at a required angle due to a hydraulic pressure derived from the OCV (not shown). In this way, since the exhaust camshaft, which is rotated in synchronization with the vane-equipped rotor 101, is rotated relative to the crankshaft, it is possible to control timing for the opening and closing of the exhaust valves (not shown).
Since the conventional valve timing control device has the construction as described above, there are problems as follows.
(1) That is, as disclosed in the JP-A-68306/1998, a biasing force of the spring 110, which is defined as the biasing means biasing the camshaft in the advanced direction, is set to be larger than the maximum torque on starting the engine or an average torque. The size of the spring 110 generating such a large biasing force must be large. It is therefore difficult to insert actually the large spring 110 into the hydraulic chamber of the valve timing control device arranged within a confined space of the engine.
(2) Since the biasing force in the advanced direction is very large, the valve timing control device defined as an actuator has two remarkable different-operation speeds between in the advanced and retarded directions, which is not negligible. The operation speed in the advanced direction can be increased due to the excess biasing force in the advanced direction, but the operation speed in the retarded direction is extremely reduced. Control characteristic of the valve timing control device becomes worse and the excess biasing force has effect, which is not negligible, on performance capabilities of the engine.
(3) Moreover, since the biasing force is very large it is difficult to perform an assembling work assembling the biasing means into the valve timing control device while controlling the biasing force. Since a rotor, which is rotated in synchronization with the camshaft, is subjected to the excess force after the assembling work, concerns are rising that the rotor is pinched.
The invention was made to solve the foregoing problems. Accordingly, it is an object of the invention to provide a valve timing control device as follows. If a case is not engaged with a rotor under conditions that an engine is stopped, it is possible to perform the engagement above at the most advanced position during one-turn of the camshaft on cranking. In this way, it is possible to prevent a deterioration of starting characteristics of the engine. At the same time, it is possible to prevent response speed differentials occurred by the biasing means biasing the camshaft in the advanced direction in the conventional device and to start the engine with stability.
In order to achieve the object of the invention, a valve timing control device mounted on an end of a camshaft having a plurality of cams opening and closing an intake or exhaust valve of an internal combustion engine to modify timing for the opening and closing of the intake or exhaust valve by way of a tappet, comprises a bias means biasing the camshaft in an advanced direction with a biasing force approximately equal to or smaller than a peak value of frictional torque produced between a cam of the camshaft and the tappet. In this way, since response speed differentials, which occur between the advanced and retarded directions due to the oversized biasing force in the conventional device, do not occur. Therefore, it is possible to prevent the deterioration of the control characteristics of the valve timing control device.
With the above arrangement, the device per se may be mounted on the camshaft corresponding to an exhaust valve of the internal combustion engine. In this way, it is possible to bias the exhaust camshaft in the advanced direction against the frictional force produced by the rotation of the cam.
With the above arrangement, the biasing force of the biasing means may be set to approximately equal to or larger than the frictional torque when an axial torque reaches a peak, the axial torque defined as a synthetic torque synthesized from the frictional torque and a cam torque being determined by a cam profile. In this way, it is possible to cancel out the frictional torque in the contact section that the rotor comes into contact with the case element at the most advanced position and to prolong the contact section.
With the above arrangement, the biasing force of the biasing means may be set to approximately equal to or larger than the frictional torque when a cam torque reaches a peak, the cam torque being determined by a cam profile. In this way, it is possible to cancel out the frictional torque in the contact section that the rotor comes into contact with the case element at the most advanced position and to prolong the contact section.
With the above arrangement, the biasing force of the biasing means may be set to approximately equal to or smaller than a peak value of the frictional torque in the range of the number of revolutions of the engine from just after cranking of the engine is started to running at stable idle, and set to approximately equal to or larger than the frictional torque when an axial torque or a cam torque reaches a peak, the axial torque defined as a synthetic torque synthesized from the frictional torque and the cam torque being determined by a cam profile. In this way, since the biasing force can be determined in the range of the number of revolutions of the engine when the maximum frictional torque is obtained, it is possible to prolong the contact section comparable to the most advanced position on starting the engine.
With the above arrangement, the number of cylinders targeted for control per a camshaft of the internal combustion engine may be three or less. Further, the biasing force of the biasing means is set to approximately equal to or smaller than the peak value of the frictional torque and is set to approximately equal to or larger than the frictional torque when the axial or cam torque reaches the peak value. In this way, since the biasing force of the biasing means can be determined depending on the frictional torque, the cam torque and the axial torque with respect to one cam, it is possible to construct a device having versatility with respect to various engines.
With the above arrangement, the number of cylinders targeted for control per a camshaft of an internal combustion engine may be four or five. Further, the biasing force of the biasing means is set to approximately equal to or smaller than the peak value of the frictional torque and is set to approximately equal to or larger than the frictional torque when the axial or cam torque reaches the peak value. In this way, since the biasing force of the biasing means can be determined depending on the frictional torque, the cam torque and the axial torque with respect to one cam, it is possible to construct a device having versatility with respect to various engines.
With the above arrangement, the number of cylinders targeted for control per a camshaft of the internal combustion engine maybe six. Further, the biasing force of the biasing means is set to approximately equal to or smaller than the peak value of the frictional torque and is set to approximately equal to or larger than the frictional torque when the axial or cam torque reaches the peak value. In this way, since the biasing force of the biasing means can be determined depending on the frictional torque, the cam torque and the axial torque with respect to one cam, it is possible to construct a device having versatility with respect to various engines.
With the above arrangement, it may further comprise a housing element having a driving force transferring means transferring a driving force from a crankshaft of the internal combustion engine to the camshaft; a rotor element fixed mounted on an end of the camshaft so as to be rotated in synchronization with the camshaft and having a plurality of vanes projected outwardly from an outer peripheral section of a boss in a radial direction of the boss; and a case element fixedly mounted on the housing element and having a plurality of shoes projected inwardly from an inner peripheral section of the case, wherein the shoes constitute a plurality of hydraulic chambers in cooperation with the vanes of the rotor element. In this way, it is possible to construct the simple device as compared with the helical piston-equipped device and to extensively reduce the cost to manufacture the device.
With the above arrangement, it may further comprise at least one biasing means, which is arranged within at least one of the hydraulic chambers comprised of the vanes of the rotor element and the shoes of the case element. In this way, it is possible to down size the device and there is a merit of allowing mounting it on various engines.
With the above arrangement, it may further comprise a lock member mating with the rotor element during a period when the rotor element comes into contact with the case element at the most advanced position due to the biasing force of the biasing means and locking the rotor element at the most advanced position. In this way, since the rotor is locked just after cranking is started on starting the engine, it is possible to prevent abnormal noise or vibration from occurring and to ensure the starting characteristics of the engine with stability.